1. Field of the Invention
The present invention relates to a monolithic optical integrated device having a high-reflectivity reflector with a small diffraction loss on facets of a semiconductor laser, and more particularly, to a multifunctional optical integrated device including a photodetector and an optical modulator monolithically integrated together with the semiconductor laser, and a manufacturing method therefor.
2. Description of the Prior Art
FIG. 1 shows an optical integrated device constructed so that a reflection multilayer 1b is opposed to the facets of a semiconductor laser 1a, which is composed of a pair of cladding layers and an active layer sandwiched between them. In the reflection multilayer 1b, semiconductor layers and low-refraction layers (air layers) are arranged alternately in the resonating direction of the laser. Each semiconductor layer has an element structure such that a waveguide layer is interposed between a pair of cladding layers.
This optical integrated device has a semiconductor multilayer structure such that an n-InP cladding layer 1d of 2-.mu.m thickness, InGaAsP quantum well layer 1e of 0.3-.mu.m thickness, p-InP cladding layer 1f of 2-.mu.m thickness, and GaInAs cap layer 1g of 0.3-.mu.m thickness are successively grown over an n-InP substrate 1c, for example. Regions for the semiconductor laser 1a and the reflection multilayer 1b are separated by subjecting a part of the semiconductor multilayer to dry or chemical etching in its thickness direction, and the reflection multilayer 1b is opposed to the facets of the semiconductor laser 1a at a predetermined distance therefrom.
The reflection multilayer 1b is composed of low-refraction layers or air layers 1h in regions removed by etching and semiconductor layers 1j each having a waveguide layer 1i between a pair of cladding layers 1d and 1f, the layers 1j and 1h being arranged alternately. The width (region width) of each of the layers 1h and 1j that constitute the reflection multilayer 1b is adjusted to, for example, 3.lambda./4, where .lambda. is the laser oscillation wavelength.
In the optical integrated device (semiconductor laser) constructed in this manner, the waveguide layer 1i of the reflection multilayer 1b has the same composition and thickness as the InGaAsP quantum well layer 1e of the semiconductor laser 1a. Theoretically, the reflectivity of the multilayer 1b is 92% if there are two pairs of semiconductor layers 1j and low-refraction layers 1h that constitute the layer 1b.
Since the waveguide layer 1i of the reflection multilayer 1b has the same composition as the InGaAsP quantum well layer 1e of the semiconductor laser 1a, however, an absorption loss of about 10,000 cm.sup.-1 is caused unless current injection is carried out, for example.
In forming a practical optical module for optical communication or the like, on the other hand, the semiconductor laser 1a is provided with a photodetector. The photodetector serves to detect the output of a laser beam generated from the laser 1a, and its output is used to stabilize the oscillating operation of the laser 1a (feedback control).
As shown in FIG. 2, a photodetector 2a of this type generally has an incident-on-the-top-surface structure such that an n-InP cladding layer 2c of 2-.mu.m thickness, InGaAsP absorption layer 2d of 2-.mu.m thickness, p-InP cladding layer 3e of 2-.mu.m thickness, and GaInAs cap layer 2f of 0.3-.mu.m thickness are successively formed over an n-InP substrate 2b. The optical module is obtained by opposing the photodetector 2a to the semiconductor laser 1a at a predetermined distance therefrom, and mounting the resulting structure in a predetermined element unit.
Also, the photodetector may be monolithically integrated on the semiconductor substrate 1c on which the semiconductor laser 1a is formed. More specifically, as shown in FIG. 3, for example, a photodetector 3a may be formed by successively stacking an n-InP cladding layer 3b, InGaAsP absorption layer 3c, p-InP cladding layer 3d, and GaInAs cap layer 3e beside the semiconductor laser 1a on the n-InP substrate 1c. Actually, the semiconductor laser 1a and the photodetector 3a are formed by dividing their respective regions in a manner such that the semiconductor multilayer on the n-InP substrate 1c is partially removed by etching.
In the optical module (optical integrated device) of this monolithic structure, however, the InGaAsP quantum well layer (laser active layer) 1e of the semiconductor laser 1a and the InGaAsP absorption layer (optical absorption layer) 3c of the photodetector 3a have the same composition and thickness, so that the laser beam emitted from the laser 1a and the photodetector 3a cannot be optically coupled with high efficiency (optical coupling efficiency is low). Since the photodetector 3a is opposed to the end face of the semiconductor laser 1a across the air layers (low-refraction layers), moreover, there is a substantial diffraction loss of light.